All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If the deceased left no heirs other than those mentioned, then her mother gets one-sixth of the estate as her fixed share due to the existence of a plurality of siblings. Allaah, The Exalted, says (what means): {And if he had brothers [or sisters], for his mother is a sixth, after any bequest he [may have] made or debt.} [Quran 4:11] The remainder of the estate is to be divided among the full brothers and full sisters by virtue of Ta‘seeb (i.e. by virtue of having a paternal relation with the deceased and not having an allotted share, so they get what is left after the allotted shares have been distributed), with the male receiving twice the amount of the female. Allaah, The Exalted, says (what means): {If there are both brothers and sisters, the male will have the share of two females.} [Quran 4:176]
Hence, the estate should be divided into fifty-four shares. The mother gets one-sixth (nine shares), each full brother gets ten shares, and each full sister gets five shares.
As for the bequest, you mentioned first that the deceased made a bequest for her nieces to receive the land after her death. This is a bequest without doubt. However, you mentioned afterwards that the bequest was executed a few months before her death. Then, this is a gift (Hibah) and not a bequest (Wasiyyah). There is a difference between a gift and a bequest; the bequest is executed after death while the gift is executed during one's life.
If the deceased authorized her nieces to take the land during her life, then she has changed the bequest into a gift. This piece of land is then considered a gift and no longer a bequest. It should be investigated as to whether or not the conditions for the validity of a gift have been fulfilled concerning it: If the deceased gave them this land while she was not in her last illness, and her nieces took possession of the land before her death, then it is a valid gift and this land is not considered part of the deceased's estate.
However, if the deceased offered them that gift while she was fatally ill and her death was feared, then this 'gift' is legally a bequest. Ibn Qudaamah said: "Gifts during the illness in which death is feared have the same legal status as the bequest." The last illness, or the fatal illness, is that which is likely to end in death. In this case, if the land does not exceed one-third of the estate, then the two nieces are entitled to it; otherwise, they should take only what is equivalent to one-third of the estate and the remaining part should be included in the deceased's estate to be divided among the heirs according to the Islamic inheritance laws; the nieces are not entitled to more than one-third of the estate unless the heirs willingly agree to that.
Allaah Knows best.